Neutrinos interact so rarely with matter that vast experiments are needed to spot them. Central to the Super-Kamiokande experiment in Japan is a huge stainless steel tank, 39 metres in diameter. It is filled with 50,000 tonnes of purified water. On the rare occasion that a neutrino interacts with the water, it converts to a charged particle that then produces a flash of light. More than 13,000 sensitive light detectors surrounding the tank watch for these flashes. They can pinpoint whether the neutrinos originated from the Earth's interior or from the heavens above.

Super-Kamiokande showed that neutrinos morph from one type to two others as they travel, like strawberry milkshake turning into chocolate or vanilla.

(Image: Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo)